Solder paste and printed circuit board

ABSTRACT

In a solder paste formed by blending an alloy powder and a flux, the alloy powder is a powder mixture formed by mixing at least one powder of a Sn—Zn based alloy and at least one powder of a Sn—Ag based alloy. The alloys powders are blended so that the composition of the powder mixture is 5-10 mass % of Zn; 0.005-1.5 mass % of Ag; optionally at least one of 0.002-1.0 mass % of Cu, 0.005-15 mass % of Bi, 0.005-15 mass % of In, and 0.005-1.0 mass % of Sb; and a remainder of Sn.

TECHNICAL FIELD

This invention relates to a solder paste for use in soldering electronicequipment and particularly to a solder paste having a solder powderwhich does not contain Pb. It also relates to a printed circuit boardhaving soldered joints formed from the solder paste.

BACKGROUND ART

The reflow soldering method (also referred to below as the reflowmethod) is particularly suitable for soldering of electronic parts. Thereflow method is a method in which a solder paste comprising a solderpowder and a flux is applied to necessary locations of a printed circuitboard (which typically have lands made of copper) by printing ordischarging through a dispenser, and after electronic parts are mountedon the coated portions of the board, the board is heated in a heatingapparatus called a reflow furnace to a temperature sufficient to meltthe solder powder in the solder paste, thereby soldering the electronicparts to the printed circuit board.

The reflow method can not only perform soldering at a large number oflocations in a single operation, but solder does not adhere tounnecessary locations, so it can perform soldering without formingbridges even with respect to electronic parts having a narrow pitch. Inaddition, the solder paste can temporarily secure the electronic parts,so it is not necessary to secure electronic parts with pins, and thesolder paste contains a flux, so an operation of applying flux isunnecessary. Therefore, the reflow method has the advantages that it canperform soldering with excellent productivity and reliability, and itcan easily cope with decreases in the size and increases in the densityof electronic parts.

A conventional solder paste which is used in the reflow method has beenprepared from a powder of a Pb—Sn alloy, a typical solder alloy whichhas been used from antiquity. A Pb—Sn alloy has a low melting point of183° C. for the eutectic composition (Pb-63Sn), so it has littleundesired thermal effect on electronic parts which are not resistant toheat. In addition, a Pb—Sn alloy has excellent solderability, and itcauses little occurrence of soldering defects such as unsolderedportions and dewetting. However, out of concern for the toxicity of Pb,in the electronic equipment industry, there is a strong demand forso-called lead-free solder which does not contain Pb.

Typical lead-free solders are Sn-based solders containing Sn as a maincomponent. Lead-free solders which are presently used include binaryalloys such as Sn-3.5Ag (melting point of 221° C.), Sn-0.7Cu (meltingpoint of 227° C.), Sn-9Zn (melting point of 199° C.), and Sn-58Bi(melting point of 139° C.), and these alloys to which one or more thirdelements such as Ag, Cu, Zn, Bi, In, Sb, Ni, Cr, Co, Fe, Mn, P, Ge, andGa are added. These alloys can be collectively referred to as Sn—Ag,Sn—Cu, Sn—Bi, or Sn—Zn based alloys.

The term “based alloy” used herein includes the alloy itself and analloy further containing one or more other elements. For example, aSn—Zn based alloy includes Sn—Zn binary alloys and Sn—Zn alloys furthercontaining at least one third element. Similarly, a Sn—Ag based alloyincludes Sn—Ag binary alloys and Sn—Ag alloys further containing atleast one third element.

A Sn—Ag based lead-free solder and a Sn—Cu based lead-free solder have amelting point of at least 220° C. even for a eutectic composition of aSn—Ag or Sn—Cu alloy. Therefore, when these are formed into a solderpaste and used in the reflow method, the peak temperature at the time ofreflow soldering becomes at least 250° C., at which temperature there isthe possibility of thermal damage to electronic parts and printedcircuit boards.

A Sn—Bi based lead-free solder has a low melting point near 139° C. atthe eutectic composition of a Sn—Bi alloy, and when it is used as asolder paste in the reflow method, the peak temperature is at most 200°C., so thermal effects on electronic parts and printed circuit boardsare avoided. However, a lead-free solder containing a large amount of Bihas a melting point which is too low, so it has problems with respect toheat resistance. Namely, when the interior of the case of a piece ofelectronic equipment reaches a high temperature during use due to heatgenerated by coils, power transistors, and the like, soldered portionsof a printed circuit board which is soldered using this lead-free solderhave a decrease in bonding strength, and there is the possibility ofpeeling occurring. In addition, a lead-free solder containing a largeamount of Bi, which is brittle, has another drawback that solderedjoints can easily peel off when they receive even a small impact.

A Sn—Zn based lead-free solder has a melting point of 199° C. for aeutectic composition of a Sn—Zn alloy. This melting point is close tothe melting point of a conventional Pb—Sn eutectic solder, so when aSn—Zn based lead-free solder is formed into a solder paste and used inthe reflow method, the peak temperature can be made 250° C. or less, andthere is little thermal effect on electronic parts and printed circuitboards. However, a solder paste using a Sn-9Zn eutectic alloy has poorsolderability with respect to portions to be soldered made of copper, sosoldering defects may occur such as unsoldered portions, where solderdoes not adhere, or dewetting, in which portions are wet by solder butrepel the solder. Such soldering defects not only reduce the bondingstrength but worsen the external appearance.

In addition, when a long period of time has passed after soldering ofcopper lands of printed circuit boards or copper leads with a solderpaste using a Sn-9Zn eutectic alloy, there are cases in which solderpeels from the interface with copper foil or copper lands due tocorrosion. Peeling of soldered joints is a cause of malfunctions inelectronic equipment.

Furthermore, tombstoning in which minute chip parts stand up at the timeof soldering can easily occur with a solder paste using a Sn-9Zneutectic alloy. If tombstoning occurs on a printed circuit board,electronic equipment in which the printed circuit board is incorporatedcannot function at all.

In order to reduce or eliminate the above-described problems of a Sn-9Zneutectic alloy, various Sn—Zn based lead-free solders to which a thirdelement is added have been proposed. For example, a solder paste using alead-free solder to which Bi is added in order to improve solderabilitysuch as one having a composition of Sn-8Zn-3Bi or Sn-8Zn-3Bi-0.1 Ag isknown in the art. In addition, a lead-free solder in which corrosionresistance is improved by adding Ag and/or Cu to an alloy having acomposition close to the Sn-9Zn eutectic composition is disclosed inJP-A 9-94687.

A Sn—Zn based lead-free solder to which one or more of Bi, Ag, and Cuare added does indeed have the effect of improving solderability andresistance to peeling compared to a Sn-9Zn eutectic alloy when used in asolder paste in the reflow method. However, soldering defects andtombstoning can still occur.

Soldering defects and tombstoning occur with a Sn—Zn based lead-freesolder because a Sn—Zn based alloy is a powder of only one type ofalloy. This alloy powder becomes rounded at the start of melting, andthe molten solder does not further wet and spread, and this causessoldering defects. In addition, this alloy powder melts within a shortlength of time during melting. Therefore, if the temperature at bothends of a chip is different at the time of reflow soldering (such as dueto a temperature gradient or a change in the temperature within afurnace), the alloy on the end which melts first pulls on the chip bysurface tension and tombstoning takes place.

In order to prevent soldering defects and tombstoning with a solderpaste using a Sn—Zn based lead-free solder, a solder paste which uses amixture of at least two lead-free solder powders (a powder mixture)having different compositions (different melting points) is disclosed inJP-A 9-277082. A solder paste using a mixture of at least two alloyseach having a eutectic composition is disclosed in JP-A 11-138282, and asolder paste comprising a Sn—Ag alloy powder mixed with another alloypowder for improving wettability is disclosed in JP-A 9-295182.

With a solder paste using a powder mixture of at least two types ofsolder powder, the alloy powder having the lower melting point meltsfirst, and the alloy powder having the higher melting point is presentin its periphery, so the alloy powder having the lower melting pointwhich melts first does not ball up, it adheres to the portion beingsoldered in this state, and good soldering is carried out. In a solderpaste using a powder mixture, the alloy powder having a lower meltingpoint melts first, and after a while, the alloy powder having the highermelting point melts. Accordingly, it takes time for the alloy powderhaving the higher melting point to completely melt, and during thisperiod, even at the other end, the alloy powder having the lower meltingpoint begins to melt, so tombstoning does not take place.

However, a solder paste using a powder mixture of conventional Sn—Znbased lead-free solders has the problem that minute solder balls aregenerated at portions being soldered at the time of reflow. In addition,with these solder pastes, the problem of peeling from portions beingsoldered made of copper after the passage of a long period of time dueto corrosion remains unresolved.

Accordingly, there is still a need for a lead-free solder paste of aSn—Zn based alloy which has good corrosion resistance and which does notproduce minute solder balls at the time of reflow.

DISCLOSURE OF THE INVENTION

This invention relates to a solder paste formed by blending an alloypowder with a flux. The alloy powder used in the present invention is apowder mixture formed by mixing at least one type of Sn—Zn based alloypowder with at least one type of Sn—Ag based alloy powder. The two typesof alloy powder are mixed so that the composition of the powder mixtureis 5-10 mass % of Zn, 0.005-1.5 mass % of Ag, and a remainder of Sn. Oneor both of these alloy powders may include one or more alloying elementsselected from Cu, Bi, In, and Sb.

Accordingly, generally, the present invention is a solder paste formedby blending an alloy powder and a flux, wherein the alloy powder is apowder mixture of at least one powder of a Sn—Zn based alloy (a Sn—Znbased alloy powder) selected from the group of the alloys Sn—Zn,Sn—Zn—Bi, Sn—Zn—In, Sn—Zn—Cu, Sn—Zn—Sb, Sn—Zn—Bi—In, Sn—Zn—Cu—Bi,Sn—Zn—Cu—In, Sn—Zn—Cu—Sb, Sn—Zn—Bi—Sb, Sn—Zn—In—Sb, Sn—Zn—Cu—Bi—In,Sn—Zn—Cu—Bi—Sb, Sn—Zn—Cu—In—Sb, Sn—Zn—Bi—In—Sb, and Sn—Zn—Cu—Bi—In—Sb,and at least one powder of a Sn—Ag based alloy (a Sn—Ag based alloypowder) selected from the group of the alloys Sn—Ag, Sn—Ag—Cu, Sn—Ag—Bi,Sn—Ag—In, Sn—Ag—Sb, Sn—Ag—Cu—Bi, Sn—Ag—Cu—In, Sn—Ag—Bi—In, Sn—Ag—Cu—Sb,Sn—Ag—Bi—Sb, Sn—Ag—In—Sb, Sn—Ag—Cu—Bi—In, Sn—Ag—Cu—Bi—Sb,Sn—Ag—Cu—In—Sb, Sn—Ag—Bi—In—Sb, and Sn—Ag—Cu—Bi—In—Sb.

These alloy powders are mixed so that the composition of the powdermixture is essentially 5-10 mass % of Zn, 0.005-1.5 mass % of Ag;optionally one or more of 0.002-1.0 mass % of Cu, 0.005-15 mass % of Bi,0.005-15 mass % of In, and 0.005-1.0 mass % of Sb; and a remainder ofSn.

Here, the “composition of the powder mixture” means the composition whenthe powder mixture is melted to form a uniform melt.

The amount of a Sn—Ag based alloy powder in the powder mixture (thetotal amount thereof when using at least two Sn—Ag based alloy powders)is preferably at most 30 mass % of the powder mixture.

The present inventors found that peeling of soldered joints, whichfrequently occurs when a Sn—Zn based lead-free solder is used to formsoldered joints on copper lands of a printed circuit board, is caused bythe formation of a Cu—Zn alloy layer in the interface between the coppersurface and the solder, as described below.

When a molten Sn-9Zn alloy adheres to the surface of copper lands of aprinted circuit board to form solder fillets during soldering, a Cu—Znalloy layer forms in the copper-solder interface due to the fact that Cuand Zn easily form an alloy. This alloy layer is exposed on the outersurface of each solder fillet which forms the foot of the fillet. Ifmoisture contacts a portion of this alloy layer which is exposed to theexterior, the Zn in the alloy layer is selectively oxidized to convertinto zinc oxide, which is the phenomenon called dezincification,resulting in a loss of bonding force of the solder joint in the affectedportion. The dezincification of the Cu—Zn alloy layer gradually proceedsfrom the exposed exterior portion of the fillet to the inside of thefillet along the copper-solder interface, and ultimately the entireCu—Zn alloy layer is dezincified, leading to peeling of the solderedjoint (fillet) from the copper land.

On the other hand, when soldering onto copper is carried out with aSn—Zn based lead-free solder containing Ag, a Cu—Zn—Ag alloy layerhaving a high melting point can form in the interface. This Cu—Zn—Agalloy layer has stable bonds, and it becomes difficult for Zn to oxidizeand dezincify. Accordingly, if soldering to copper is performed using aSn—Zn based lead-free solder to which Ag is added (such as a Sn—Zn—Agalloy), corrosion does not occur even over long periods, and solderedjoints do not peel off. If Bi is further added to this alloy, it becomesdifficult for soldering defects to occur. However, a solder paste usingthese alloy powders cannot prevent the formation of minute balls.

The reason why minute solder balls are formed with a solder paste usinga Sn—Zn—Ag based alloy powder such as Sn—Zn—Ag or Sn—Zn—Ag—Bi is becausea high melting point Zn—Ag intermetallic compound is already formed inthese alloy powders. An alloy powder in which a Zn—Ag intermetalliccompound is present has a high liquidus temperature, so it does notreadily flow until it completely melts. When a solder paste is appliedto a printed circuit board or when it undergoes preheating in a reflowfurnace, it sags and flows outward with the flow of the flux, and itprotrudes outward from the portion being soldered. At this time, aSn—Zn—Ag based lead-free solder in which a Zn—Ag intermetallic compoundis present does not readily flow at the time of reflow, so even if boththe solder which extends to the exterior of the portion being solderedand the solder on the portion being soldered melt, the molten solder onthe portion being soldered cannot pull in the molten solder extending tothe exterior, so the solder on the exterior remains there and becomessolder balls.

According to the present invention, a solder paste using a powdermixture formed by mixing a Sn—Zn based alloy powder and a Sn—Ag basedalloy powder does not have a Zn—Ag intermetallic compound present ineither alloy, so its melting point is not so high, and it can readilyflow at the time of melting. Accordingly, at the time of soldering, evenif solder which extends to the exterior of the portion being solderedmelts there, it is pulled back in by the solder which melts on theportion being soldered, so it does not become minute balls. With thissolder paste comprising a powder mixture, a Zn—Ag intermetallic compoundis formed when the two types of alloy powder melt together, but theZn—Ag intermetallic compound which is formed after melting is in no wayconnected to the formation of minute balls. Moreover, when solderingonto a copper surface is performed, it alloys with copper present in theportion being soldered, and it forms a Cu—Zn—Ag intermetallic compoundwhich has the effect of preventing corrosion, so corrosion resistance isfurther increased.

One or more third elements selected from Cu, Bi, In, and Sb may be addedto one or both of a Sn—Zn based alloy powder and a Sn—Ag based alloypowder. The addition of Cu increases corrosion resistance, and it isalso effective at improving reflow properties by lowering the meltingtemperature. The addition of Bi and/or In is effective at lowering themelting point of the alloy powder and improving solderability. Sbsuppresses oxidation of Zn, so it has the effect of improving corrosionresistance.

The present invention also relates to a soldered joint formed bysoldering using the above-described solder paste and a printed circuitboard having a soldered joint formed by soldering using theabove-described solder paste. Soldering is preferably carried out withrespect to copper.

In the present invention, “printed circuit board” means both a printedcircuit board (printed wiring board) on which electronic parts such assemiconductor packages are mounted and a circuit substrate on whichsemiconductor chips are mounted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph showing the state of minute ball formation in anembodiment, and

FIG. 2 is a photograph showing the state of minute ball formation in acomparative example.

DETAILED EXPLANATION OF THE INVENTION

A solder paste according to the present invention uses a Sn—Zn basedalloy powder and a Sn—Ag based alloy powder which are mixed together.

The Sn—Zn based alloy powder is a powder of at least one alloy selectedfrom the group of Sn—Zn, Sn—Zn—Bi, Sn—Zn—In, Sn—Zn—Cu, Sn—Zn—Sb,Sn—Zn—Bi—In, Sn—Zn—Cu—Bi, Sn—Zn—Cu—In, Sn—Zn—Cu—Sb, Sn—Zn—Bi—Sb,Sn—Zn—In—Sb, Sn—Zn—Cu—Bi—In, Sn—Zn—Cu—Bi—Sb, Sn—Zn—Cu—In—Sb,Sn—Zn—Bi—In—Sb, and Sn—Zn—Cu—Bi—In—Sb alloys.

The Sn—Ag based alloy powder is a powder of at least one alloy selectedfrom the group of Sn—Ag, Sn—Ag—Cu, Sn—Ag—Bi, Sn—Ag—In, Sn—Ag—Sb,Sn—Ag—Cu—Bi, Sn—Ag—Cu—In, Sn—Ag—Bi—In, Sn—Ag—Cu—Sb, Sn—Ag—Bi—Sb,Sn—Ag—In—Sb, Sn—Ag—Cu—Bi—In, Sn—Ag—Cu—Bi—Sb, Sn—Ag—Cu—In—Sb,Sn—Ag—Bi—In—Sb, and Sn—Ag—Cu—Bi—In—Sb alloys.

In order to improve reflow properties, it is preferable for the meltingtemperature of the solder powder to be as low as possible. For thisreason, the mixing ratio of the Sn—Zn based alloy powder is made suchthat the Zn content of the powder mixture (as stated above, the Zncontent when the powder mixture is melted) is made 5-10 mass % so as tobe near the eutectic composition. If the Zn content of the powdermixture is smaller than 5 mass % or larger than 10 mass %, the liquidustemperature of the solder increases, and the soldering temperatureincreases. The Zn content of the powder mixture is preferably 6-10 mass% and more preferably 7-9 mass %.

The mixing ratio of the Sn—Ag based alloy powder is made such that theAg content in the powder mixture (as stated above, the Ag content whenthe powder mixture is melted) is 0.005-1.5 mass %. If the Ag content issmaller than 0.005 mass %, the effect of improving corrosion resistanceafter reflow is not sufficient. On the other hand, if the Ag contentexceeds 1.5 mass %, a large amount of solder balls are formed at thetime of reflow soldering, and reflow properties are deteriorated. The Agcontent of the powder mixture is preferably 0.01-1.0 mass % and morepreferably 0.05-0.5 mass %.

When each of the Sn—Ag and Sn—Zn based alloy powders is a powder of abinary alloy (i.e., a Sn—Ag alloy and a Sn—Zn alloy), it is preferablethat the powder mixture have an Ag content of not greater than 0.3 mass% in order to improve reflow properties.

When Cu is added to a Sn—Zn based alloy powder, a powder of at least onealloy selected from the group of Sn—Zn—Cu, Sn—Zn—Cu—Bi, Sn—Zn—Cu—In,Sn—Zn—Cu—Sb, Sn—Zn—Cu—Bi—In, Sn—Zn—Cu—Bi—Sb, Sn—Zn—Cu—In—Sb, andSn—Zn—Cu—Bi—In—Sb alloys can be used. When Cu is added to a Sn—Ag basedalloy powder, a powder of at least one alloy selected from the group ofSn—Ag—Cu, Sn—Ag—Cu—Bi, Sn—Ag—Cu—In, Sn—Ag—Cu—Sb, Sn—Ag—Cu—Bi—In,Sn—Ag—Cu—Bi—Sb, Sn—Ag—Cu—In—Sb, and Sn—Ag—Cu—Bi—In—Sb alloys can beused.

When using at least one of these Cu-containing alloy powders, the mixingratio is such that the content of Cu in the powder mixture is 0.002-1.0mass %. If the Cu content of the powder mixture is smaller than 0.002mass %, an effect of lowering the melting temperature of the powdermixture is not achieved. On the other hand, if the Cu content of thepowder mixture exceeds 1.0 mass %, the liquidus temperature of thepowder mixture increases, and the effect of lowering the meltingtemperature is suppressed. The Cu content is preferably 0.005-0.5 mass %and more preferably 0.01-0.3 mass %.

When Bi and/or In is added to the alloy powder, as a Sn—Zn based alloy,at least one of Sn—Zn—Bi, Sn—Zn—In, Sn—Zn—Bi—In, Sn—Zn—Cu—Bi,Sn—Zn—Cu—In, Sn—Zn—Bi—Sb, Sn—Zn—In—Sb, Sn—Zn—Cu—Bi—In, Sn—Zn—Cu—Bi—Sb,Sn—Zn—Cu—In—Sb, Sn—Zn—Bi—In—Sb, and Sn—Zn—Cu—Bi—In—Sb alloys can beused, and as a Sn—Ag based alloy, at least one of Sn—Ag—Bi, Sn—Ag—In,Sn—Ag—Cu—Bi, Sn—Ag—Cu—In, Sn—Ag—Bi—In, Sn—Ag—Bi—Sb, Sn—Ag—In—Sb,Sn—Ag—Cu—Bi—In, Sn—Ag—Cu—Bi—Sb, Sn—Ag—Cu—In—Sb, Sn—Ag—Bi—In—Sb, andSn—Ag—Cu—Bi—In—Sb alloys can be used.

When using at least one of these Bi- and/or In-containing alloy powders,the mixing ratio is such that the content of Bi and In of the powdermixture are each 0.005-15 mass %. If the content of either Bi or In issmaller than 0.005 mass %, an effect of reducing the melting temperatureis not achieved. If the content thereof exceeds 15 mass %, the effect ofthe Sn—Bi eutectic temperature of 139° C. or the effect of the Sn—Ineutectic temperature of 117° C. becomes large, so the heat resistance ofthe solder decreases, and there are cases in which the bonding strengthof soldered joints decreases due to heat generated by heat generatingparts. The content is preferably 0.01-5 mass %. When both Bi and In areadded, the combined content of Bi and In in the powder is preferably0.005-15 mass %.

When Sb is added to the alloy powder, a Sn—Zn based alloy can be atleast one selected from the group of Sn—Zn—Sb, Sn—Zn—Cu—Sb, Sn—Zn—Bi—Sb,Sn—Zn—In—Sb, Sn—Zn—Cu—Bi—Sb, Sn—Zn—Cu—In—Sb, Sn—Zn—Bi—In—Sb alloys, andSn—Zn—Cu—Bi—In—Sb, and a Sn—Ag based alloy can be at least one selectedfrom the group of Sn—Ag—Sb, Sn—Ag—Cu—Sb, Sn—Ag—Bi—Sb, Sn—Ag—In—Sb,Sn—Ag—Cu—Bi—Sb, Sn—Ag—Cu—In—Sb, Sn—Ag—Bi—In—Sb, and Sn—Ag—Cu—Bi—In—Sballoys.

When at least one of these Sb-containing alloy powders are used, themixing ratio is such that the Sb content in the powder mixture is0.005-1.0 mass %. If the Sb content of the powder mixture is less than0.005 mass %, the effect of improving corrosion resistance is notachieved. On the other hand, if the Sb content exceeds 1.0 mass %, ithas an adverse effect on wettability. The Sb content of the powdermixture is preferably 0.01-0.1 mass %.

In the present invention, the combination of a binary Sn—Zn alloy powderand a binary Sn—Ag alloy powder may be used. However, preferably atleast one of the Sn—Zn based alloy powder and the Sn—Ag based alloypowder is a ternary or higher alloy powder containing at least one ofthe above-described alloying elements, since the reflow properties areimproved and the formation of minute balls is reduced. The reason forthis is thought to be that a ternary or higher alloy has a lower meltingpoint than a binary alloy, and the separation between the solidustemperature and the liquidus temperature increases.

As explained below, the mixing ratio of the Sn—Zn based alloy powder andthe Sn—Ag based alloy powder is preferably such that the proportion ofthe Sn—Ag based alloy powder in the powder mixture is at most 30 mass %.

When the reflow temperature of the solder paste is made at most 230° C.in consideration of thermal damage to electronic parts, the meltingpoint of the Sn—Ag based alloy, which has a higher melting point thanthe Sn—Zn based alloy, may be at most 250° C. Even if the Sn—Ag basedalloy has a melting point of 250° C., a powder of this alloy can melt ata reflow temperature of 230° C. because it is present in a solder pastein the form of the powder mixture. In the case of the powder mixture, ifthe Sn—Zn based alloy powder having a lower melting temperature firstmelts at a reflow temperature of at most 230° C., a melt of the lowermelting point alloy which has melted spreads into the higher meltingpoint Sn—Ag based alloy powder to cause the higher melting point alloypowder to melt. However, if the melting point of the higher meltingpoint Sn—Ag alloy powder is too high, a long time is required until thehigher melting point alloy powder melts by the above-describedmechanism. Therefore, a melting point of at most 250° C. is suitable forthe higher melting point Sn—Ag alloy powder. In order to make themelting point of the Sn—Ag alloy powder at most 250° C., the maximumadded amount of Ag is made 5 mass %. On the other hand, in the presentinvention, the Ag content of the powder mixture in the present inventionis at most 1.5 mass %. In order to make the Ag content of a powdermixture at most 1.5 mass % when the Ag content of the Sn—Ag alloy powderis at most 5 mass %, the proportion of the Sn—Ag based alloy powder ismade at most 30 mass %.

A solder paste according to the present invention comprises a Sn—Znbased alloy powder and a Sn—Ag based alloy powder which are mixed in theabove proportions. The average particle diameter of the alloy powderswhich are used is typically in the range of 10-50 micrometers. Thesepowders can be manufactured by the gas atomizing method, for example.

The alloy powder mixture is blended with a flux to prepare a solderpaste. Preferably, the flux which is used is one which hasconventionally been used with a Sn—Zn based alloy powder to manufacturea solder paste. An example of such a flux is a rosin flux which containsan activator (e.g., an amine hydrobromide) and a thixotropic agent(e.g., hardened castor oil), although other fluxes may be employed. Theflux may further contain one or more additives such as halogenatedaliphatic or aromatic compound and surfactant, as described in JP-A10-175092.

The mixing ratio of the alloy powder and the flux is selected so as toobtain a solder paste having a consistency suitable for application. Forexample, the alloy powder may constitute 80-95 mass %, preferably 85-95mass %, of the solder paste.

A soldered joint or a printed circuit board formed by a reflow methodusing a solder paste according to the present invention is formed bysoldering with a solder paste using a powder mixture of a Sn—Zn basedalloy powder and a Sn—Ag based alloy powder, so it has excellentreliability without soldering defects or formation of minute balls.

EXAMPLES

Sn—Zn based alloy powders and Sn—Ag based alloy powders having thecompositions shown in Tables 1 and 2 were obtained by the gas atomizingmethod. The average particle diameter of these alloy powders was in therange of 10-50 micrometers. These alloy powders were mixed in the mixingratios shown in Tables 1 and 2, and the resulting powder mixture wasblended with a polymerized rosin-based flux which containeddiphenylguanidine HBr as an activator and hardened castor oil as athixotropic agent in alpha-terpineol as a solvent to prepare a solderpaste. Tables 1 and 2 also shows the composition of the alloy powdermixture (mixed powder).

The following properties of the solder pastes were investigated.

[Corrosion Resistance]

Test piece: A tough pitch copper plate measuring 0.3 mm×10 mm×15 mm wasimmersed to a depth of 15 mm in a molten solder alloy prepared byheating the powder mixture used for the solder paste to 250° C. toprepare a soldered test piece.

Test method: The soldered test piece was left for 1000 hours in athermo-hygrostat having a temperature of 85° C. and a relative humidityof 85%, then it was immobilized in an epoxy potting resin, and the crosssection was polished. The cross section was observed with a scanningelectron microscope and an energy dispersive elementary analysisapparatus to determine whether there was any corrosion by oxidation inthe interface of the soldered joint.

Results: The case in which the formation of an oxide layer due tocorrosion by oxidation in the interface of the soldered joint was notobserved or in which there was little formation of an oxide layer isindicated as good (acceptable), and the case in which there was muchformation of an oxide layer due to corrosion by oxidation in theinterface of the soldered joint or in which peeling of the interface wasobserved is indicated as poor (unacceptable).

[Reflow Properties]

Test piece: A solder paste was applied by printing using a metal screenhaving a thickness of 0.15 mm to a copper wiring printed circuit boardhaving a QFP pattern with a pitch of 0.65 mm. After a QFP was mounted onthe solder paste-coated circuit board, soldering was carried out byheating the board in a reflow furnace such that the peak temperature was210-220° C. to obtain a test piece.

Results: The surface and the periphery of the soldered joint wereobserved for the extent of formation of minute balls. The standard forevaluation was follows. Excellent: extremely few minute balls, good: fewminute balls, fair: some occurrence of minute balls, poor: muchoccurrence of minute balls. Those that were evaluated to be poor couldnot be used.

FIG. 1 is a photograph showing the state of minute ball formation inExample 13, and FIG. 2 is a photograph showing the state of minute ballformation in Comparative Example 5. From the photographs of the state ofminute ball formation, it can be seen that there was no formation ofminute balls in the example, whereas a large number of minute balls wereformed in the periphery of the soldered joint in the comparativeexample.

[QFP Bonding Strength]

Test piece: The test piece which was used for evaluation of reflowpropertied and which was a copper wire printed circuit board to whichQFP was soldered by the reflow method was also used to evaluate for thebonding strength of the soldered QFP.

Test method: The test piece was left for 1000 hours in athermo-hygrostat having a temperature of 85° C. and a relative humidityof 85%. Thereafter, a tensile test was carried out at by pulling thetest piece a sloping angle of 45° using a hook which had been engaged ata soldered lead of the QFP to determine the bonding strength, which wascompared to the initial bonding strength measured in the same manner.

Results: A decrease in bonding strength was observed as corrosion byoxidation in the interface of the soldered joint proceeded, and thefailure mode became peeling at the interface.

As can be seen from Tables 1 and 2, a solder paste prepared using apowder mixture of a Sn—Zn based alloy powder and a Sn—Ag based alloypowder according to the present invention had good corrosion resistanceand good reflow properties. In addition, a soldered joint and a printedcircuit board formed by soldering using this solder paste have goodcorrosion resistance, so they maintain a good bonding strength evenafter being left for 1000 hours under high temperature, high humidityconditions of 80° C. and a relative humidity of 85%, so they canwithstand use over long periods.

Even though a solder paste according to present invention is a Sn—Znbased solder paste, it has good solderability, corrosion resistance, andreflow properties, so it provides the excellent effect not provided byconventional Sn—Zn baseds that defects such as unsoldered portions anddewetting do not occur at the time of soldering, an increase in lifespancan be achieved over long periods of use, and the formation of minuteballs is extremely small. As a result, it can provide reliable solderedjoints and printed circuit boards without soldering defects or minuteballs.

TABLE 1 Mixing ratio (mass %) Sn—Zn Sn—Ag Composition of alloy powder(mass %) based based Example Sn—Zn based alloy Sn—Ag based alloy alloyalloy  1 Sn—11Zn Sn—3.5Ag 91.0 9.0  2 Sn—9Zn Sn—3.5Ag 99.7 0.3  3 Sn—5ZnSn—3.5Ag 99.7 0.3  4 Sn—9Zn Sn—3.5Ag 97.1 2.9  5 Sn—9Zn Sn—3.5Ag 99.850.15  6 Sn—9Zn Sn—4Ag—0.5Cu 99.6 0.4  7 Sn—9Zn Sn—3Ag—0.5Cu 96.7 3.3  8Sn—9Zn Sn—3.5Ag—1Cu 71.0 29.0  9 Sn—9Zn Sn—4Ag—0.5Cu 75.0 25.0 10Sn—9Zn—1Cu Sn—3.5Ag—1Cu 80.0 20.0 11 Sn—9Zn Sn—3.5Ag—2Bi 99.7 0.3 12Sn—9Zn Sn—3.5Ag—2Bi 71.0 29.0 13 Sn—8Zn—3Bi Sn—3.5Ag 97.1 2.9 14 Sn—9ZnSn—1Ag—57Bi 90.0 10.0 15 Sn—8Zn—3Bi Sn—3.5Ag—2Bi 98.6 1.4 16 Sn—8Zn—3InSn—3.5Ag 97.1 2.9 17 Sn—9Zn Sn—1Ag—50In 90.0 10.0 18 Sn—9Zn Sn—3.5Ag—2In99.7 0.3 19 Sn—8Zn—3Bi—0.1Cu Sn—3Ag—0.5Cu 96.7 3.3 20 Sn—9ZnSn—2Ag—0.5Cu—15Bi 75.0 25.0 21 Sn—8Zn—12Bi Sn—2Ag—0.5Cu—25Bi 80.0 20.022 Sn—9Zn Sn—3Ag—0.7Cu—5In 96.7 3.3 23 Sn—8Zn—12In Sn—2Ag—0.5Cu—25In80.0 20.0 24 Sn—8Zn—3In—0.1Cu Sn—3Ag—0.5Cu 96.7 3.3 25 Sn—9ZnSn—3.5Ag—0.5Bi—8In 97.1 2.9 26 Sn—9Zn Sn—3.5Ag—0.5Bi—8In 71.0 29.0 27Sn—8Zn—3In Sn—3.5Ag—2Bi 97.1 2.9 28 Sn—8Zn—3Bi—3In Sn—3.5Ag—0.5Bi—8In97.1 2.9 29 Sn—8Zn—3Bi Sn—3Ag—0.7Cu—1In 83.3 16.7 30 Sn—9ZnSn—3Ag—0.7Cu—1Bi—2.5In 93.0 7.0 31 Sn—8Zn—3Bi—3In—0.1Cu Sn—3Ag—0.5Cu96.7 3.3 32 Sn—9Zn Sn—3.4Ag—3Sb 95.0 5.0 33 Sn—9Zn Sn—0.3Ag—0.7Cu—0.3Sb98.3 1.7 Properties Composition of mixed powder (mass %) QFP bondstrength (N) Example Sn Ag Zn Cu Bi In Sb CR.* Reflow Initial After 1000hr  1 bal. 0.3 10.0 Good Fair 25.9 23.2  2 bal. 0.01 9.0 Good Good 26.416.9  3 bal. 0.01 5.0 Good Fair 25.3 16.4  4 bal. 0.1 8.7 Good Good 28.325.0  5 bal. 0.005 9.0 Good Good 26.1 15.1  6 bal. 0.02 9.0 0.002 GoodExel. 25.8 17.1  7 bal. 0.1 8.7 0.02 Good Exel. 28.6 24.3  8 bal. 1 6.40.3 Good Fair 29.5 26.8  9 bal. 1 6.8 0.1 Good Fair 29.2 26.5 10 bal.0.7 7.2 1.0 Good Fair 28.5 25.5 11 bal. 0.01 9.0 0.006 Good Exel. 26.117.3 12 bal. 1 6.4 0.6 Good Fair 29.9 25.3 13 bal. 0.1 7.8 2.9 GoodExel. 31.5 20.3 14 bal. 0.1 8.1 5.7 Good Exel. 33.1 19.6 15 bal. 0.057.9 3.0 Good Exel. 30.6 18.9 16 bal. 0.1 7.8 2.9 Good Exel. 30.2 23.4 17bal. 0.1 8.1 5.0 Good Exel. 30.8 21.1 18 bal. 0.01 9.0 0.0 Good Exel.25.7 17.5 19 bal. 0.1 7.7 0.1 2.9 Good Exel. 30.3 23.4 20 bal. 0.5 6.80.1 3.8 Good Good 32.6 23.5 21 bal. 0.4 6.4 0.1 15.0 Good Exel. 28.122.0 22 bal. 0.1 8.7 0.02 0.17 Good Exel. 29.2 22.4 23 bal. 0.4 6.4 0.115.0 Good Exel. 27.3 22.5 24 bal. 0.1 7.7 0.1 2.9 Good Exel. 28.6 21.925 bal. 0.1 8.7 0.01 0.2 Good Exel. 29.4 23.7 26 bal. 1 6.4 0.1 2.3 GoodExel. 29.3 27.2 27 bal. 0.1 7.8 0.1 2.9 Good Exel. 30.5 23.1 28 bal. 0.17.8 2.9 3.1 Good Exel. 32.7 20.8 29 bal. 0.5 6.7 0.1 2.5 0.2 Good Exel.29.8 23.8 30 bal. 0.2 8.4 0.05 0.07 0.2 Good Exel. 28.5 21.6 31 bal. 0.17.7 0.1 2.9 2.9 Good Exel. 32.6 24.0 32 bal. 0.17 8.6 0.15 Good Good27.9 24.9 33 bal. 0.005 8.8 0.01 0.005 Good Good 25.8 17.6 CR.*:Corrosion Resistance

TABLE 2 Mixing ratio (mass %) Sn—Zn Sn—Ag Composition of alloy powder(mass %) based based Sn—Zn based alloy Sn—Ag based alloy alloy alloyExample 34 Sn—9Zn Sn—3.4Ag—0.7Cu—0.3Sb 95.0 5.0 35 Sn—8Zn—3BiSn—3.4Ag—3Sb 98.3 1.7 36 Sn—8Zn—3Bi Sn—3Ag—3Bi—1Sb 95.0 5.0 37 Sn—11ZnSn—3.5Ag—15In—0.3Sb 70.0 30.0 38 Sn—8Zn—3Bi—0.1Cu—0.05Sb Sn—3Ag—0.5Cu95.0 5.0 39 Sn—8Zn—3Bi Sn—3.5Ag—1Cu—0.5Bi—4Sb 97.5 2.5 40Sn—8Zn—3Bi—0.8Sb Sn—1Ag—1Cu—3Sb 95.0 5.0 41 Sn—8Zn—3In—0.05SbSn—3Ag—0.5Cu 95.0 5.0 42 Sn—8Zn—3In—0.1Cu—0.05Sb Sn—3Ag—0.5Cu 95.0 5.044 Sn—9Zn—0.05Sb Sn—3.5Ag—0.5Bi—8In 95.0 5.0 43 Sn—9ZnSn—3.5Ag—0.5Bi—8In—0.5Sb 95.0 5.0 45 Sn—8Zn—3BiSn—3.5Ag—0.7Cu—1.5In—1.5Sb 98.5 1.5 46 Sn—8Zn—3BiSn—3Ag—0.7Cu—1Bi—2.5In—0.5S 95.0 5.0 47 Sn—9Zn—0.1Cu—0.05SbSn—3.5Ag—0.5Bi—8In 95.0 5.0 48 Sn—8Zn—3Bi—3In—0.05Sb Sn—3Ag—0.5Cu 95.05.0 49 Sn—8Zn—3Bi—3In—0.1Cu—0.05Sb Sn—3Ag—0.5Cu 95.0 5.0 Comparative  1Sn—9Zn — 0.0 100.0  2 Sn—8Zn—3Bi — 0.0 100.0  3 Sn—9Zn—0.01Ag — 0.0100.0  4 Sn—9Zn—0.1Ag — 0.0 100.0  5 Sn—9Zn—1Ag — 0.0 100.0  6Sn—8Zn—3Bi—0.1Ag — 0.0 100.0  7 Sn—14Zn Sn—3.5Ag 99.7 0.3  8 Sn—3ZnSn—3.5Ag 99.7 0.3  9 Sn—9Zn Sn—3Ag 99.9 0.1 10 Sn—11Zn Sn—7Ag 75.0 25.011 Sn—9Zn Sn—3.5Ag 30.0 70.0 12 Sn—9Zn Sn—5Ag 60.0 40.0 13 Sn—9Zn—1CuSn—3.5Ag—1.5Cu 75.0 25.0 14 Sn—9Zn Sn—3Ag—3Sb 99.9 0.1 15 Sn—8Zn—3BiSn—3Sb 96.7 3.3 16 Sn—8Zn—3Bi—0.8Sb Sn—1Ag—1Cu—5Sb 90.0 10.0 PropertiesComposition of mixed powder (mass %) QFP bond strength (N) Sn Ag Zn CuBi In Sb CR.* Reflow Initial After 1000 hr Example 34 bal. 0.17 8.600.04 0.015 Good Good 26.4 24.2 35 bal. 0.06 7.8 2.9 0.05 Good Exel. 29.126.0 36 bal. 0.15 7.6 3.0 0.05 Good Exel. 31.2 27.4 37 bal. 1.1 7.7 4.50.09 Good Exel. 29.3 25.8 38 bal. 0.15 7.6 0.1 2.9 0.05 Good Exel. 31.826.6 39 bal. 0.09 7.8 0.03 0.7 0.1 Good Exel. 29.5 26.5 40 bal. 0.05 7.60.05 2.9 0.9 Good Good 30.4 27.8 41 bal. 0.15 7.6 0.03 2.9 0.05 GoodExel. 28.3 24.5 42 bal. 0.15 7.6 0.1 2.9 0.05 Good Exel. 29.8 25.7 44bal. 0.18 8.6 0.03 0.4 0.05 Good Exel. 28.1 23.9 43 bal. 0.18 8.6 0.030.4 0.03 Good Exel. 29.0 25.0 45 bal. 0.05 7.9 0.01 3.0 0.02 0.02 GoodExel. 29.6 22.3 46 bal. 0.15 7.6 0.04 2.9 0.13 0.03 Good Exel. 29.9 25.147 bal. 0.18 8.6 0.1 0.03 0.4 0.05 Good Exel. 29.5 24.7 48 bal. 0.15 7.60.03 2.9 2.9 0.05 Good Exel. 32.6 27.1 49 bal. 0.15 7.6 0.1 2.9 2.9 0.05Good Exel. 33.0 26.5 Comparative  1 bal. 9 Poor Exel. 25.7 10.5  2 bal.8 3.0 Poor Exel. 29.3 9.1  3 bal. 0.01 9 Good Poor 26.2 17.3  4 bal. 0.19 Good Poor 28.4 24.3  5 bal. 1 9 Good Poor 29.6 26.7  6 bal. 0.1 8 3Good Poor 31.7 21.1  7 bal. 0.01 14 Good Poor 27.6 24.4  8 bal. 0.01 3.0Good Poor 25.2 19.8  9 bal. 0.003 9.0 Poor Good 26.4 10.9 10 bal. 1.88.3 Good Poor 25.5 22.6 11 bal. 2.5 2.7 Good Poor 25.6 23.4 12 bal. 25.4 Good Poor 26.3 23.8 13 bal. 0.9 6.8 1.1 Good Poor 27.7 23.8 14 bal.0.003 9.0 0.003 Poor Fair 26.1 12.3 15 bal. 7.7 2.9 0.1 Poor Good 31.311.2 16 bal. 0.05 7.6 0.05 2.9 1.2 Good Poor 30.3 27.1 CR.*: CorrosionResistance

1. A solder paste comprising an alloy powder blended with a flux,wherein the alloy powder is a powder mixture consisting of a pluralityof powders each of which is a Sn—Zn based alloy powder not containing Agor a Sn—Ag based alloy not containing Zn, each of the plurality ofpowders is selected from a Sn—Zn binary alloy powder, a Sn—Zn—Bi ternaryalloy powder, a Sn—Zn—In ternary alloy powder, a Sn—Zn—Cu ternary alloypowder, a Sn—Zn—Sb ternary alloy powder, a Sn—Zn—Bi—In quaternary alloypowder, a Sn—Zn—Cu—Bi quaternary alloy powder, a Sn—Zn—Cu—In quaternaryalloy powder, a Sn—Zn—Cu—Sb quaternary alloy powder, a Sn—Zn—Bi—Sbquaternary alloy powder, a Sn—Zn—In—Sb quaternary alloy powder, aSn—Zn—Cu—Bi—In five-element alloy powder, a Sn—Zn—Cu—Bi—Sb five-elementalloy powder, a Sn—Zn—Cu—In—Sb five-element alloy powder, aSn—Zn—Bi—In—Sb five-element alloy powder, a Sn—Zn—Cu—Bi—In—Sbsix-element alloy powder, a Sn—Ag binary alloy powder, a Sn—Ag—Cuternary alloy powder, a Sn—Ag—In ternary alloy powder, a Sn—Ag—Sbternary alloy powder, a Sn—Ag—Cu—Bi quaternary alloy powder, aSn—Ag—Cu—In quaternary alloy powder, a Sn—Ag—Bi—In quaternary alloypowder, a Sn—Ag—Cu—Sb quaternary alloy powder, a Sn—Ag—Bi—Sb quaternaryalloy powder, a Sn—Ag—In—Sb quaternary alloy powder, a Sn—Ag—Cu—Bi—Infive-element alloy powder, a Sn—Ag—Cu—Bi—Sb five-element alloy powder, aSn—Ag—Cu—In—Sb five-element alloy powder, a Sn—Ag—Bi—In—Sb five-elementalloy powder, and a Sn—Ag—Cu—Bi—In—Sb six-element alloy powder, and thecomposition of the powder mixture consists of 5-10 mass % of Zn,0.005-1.5 mass % of Ag, one or more selected from the group consistingof 0.002-1.0 mass % of Cu, 0.005-15 mass % of Si, 0.005-15 mass % of In,and 0.005-1.0 mass % of Sb, and a remainder of Sn.
 2. A solder paste asclaimed in claim 1 wherein each powder in the powder mixture is selectedfrom a Sn—Zn binary alloy powder, a Sn—Zn—Cu ternary alloy powder, aSn—Ag binary alloy powder, and a Sn—Ag—Cu ternary alloy powder, and thecomposition of the powder mixture consists of 5-10 mass % of Zn,0.005-1.5 mass % of Ag, 0.002-1.0 mass % of Cu, and a remainder of Sn.3. A solder paste as claimed in claim 1 wherein each powder in thepowder mixture is selected from a Sn—Zn binary alloy powder, a Sn—Zn—Biternary alloy powder, and a Sn—Ag binary alloy powder, and thecomposition of the powder mixture consists of 5-10 mass % of Zn,0.005-1.5 mass % of Ag, 0.005-15 mass % of Bi, and a remainder of Sn. 4.A solder paste as claimed in claim 1 wherein each powder in the powdermixture is selected from a Sn—Zn binary alloy powder, a Sn—Zn—In ternaryalloy powder, a Sn—Ag binary alloy powder, and a Sn—Ag—In ternary alloypowder, and the composition of the powder mixture consists of 5-10 mass% of Zn, 0.005-1.5 mass % of Ag, 0.005-15 mass % of In, and a remainderof Sn.
 5. A solder paste as claimed in claim 1 wherein each powder inthe powder mixture is selected from a Sn—Zn binary alloy powder, aSn—Zn—Bi ternary alloy powder, a Sn—Zn—Cu ternary alloy powder, aSn—Zn—Cu—Bi quaternary alloy powder, a Sn—Ag binary alloy powder, aSn—Ag—Cu ternary alloy powder, and a Sn—Ag—Cu—Bi quaternary alloypowder, and the composition of the powder mixture consists of 5-10 mass% of Zn, 0.005-1.5 mass % of Ag, 0.002-1.0 mass % of Cu, 0.005-15 mass %of Bi, and a remainder of Sn.
 6. A solder paste as claimed in claim 1wherein each powder in the powder mixture is selected from a Sn—Znbinary alloy powder, a Sn—Zn—In ternary alloy powder, a Sn—Zn—Cu ternaryalloy powder, a Sn—Zn—Cu—In quaternary alloy powder, a Sn—Ag binaryalloy powder, a Sn—Ag—Cu ternary alloy powder, a Sn—Ag—In ternary alloypowder, and a Sn—Ag—Cu—In quaternary alloy powder, and the compositionof the powder mixture consists of 5-10 mass % of Zn, 0.005-1.5 mass % ofAg, 0.002-1.0 mass % of Cu, 0.005-15 mass % of In, and a remainder ofSn.
 7. A solder paste as claimed in claim 1 wherein each powder in thepowder mixture is selected from a Sn—Zn binary alloy powder, a Sn—Zn—Biternary alloy powder, a Sn—Zn—In ternary alloy powder, a Sn—Zn—Bi—Inquaternary alloy powder, a Sn—Ag binary alloy powder, a Sn—Ag—In ternaryalloy powder, and a Sn—Ag—Bi—In quaternary alloy powder, and thecomposition of the powder mixture consists of 5-10 mass % of Zn,0.005-1.5 mass % of Ag, 0.005-15 mass % of Bi, 0.005 is mass % of In,and a remainder of Sn.
 8. A solder paste as claimed in claim 1 whereineach powder in the powder mixture is selected from a Sn—Zn binary alloypowder, a Sn—Zn—Bi ternary alloy powder, a Sn—Zn—In ternary alloypowder, a Sn—Zn—Cu ternary alloy powder, a Sn—Zn—Cu—Bi quaternary alloypowder, a Sn—Zn—Cu—In quaternary alloy powder, a Sn—Zn—Bi—In quaternaryalloy powder, a Sn—Zn—Cu—Bi—In five-element alloy powder, a Sn—Ag binaryalloy powder, a Sn—Ag—Cu ternary alloy powder, a Sn—Ag—In ternary alloypowder, a Sn—Ag—Cu—Bi quaternary alloy powder, a Sn—Ag—Cu—In quaternaryalloy powder, a Sn—Ag—Bi—In quaternary alloy powder, and aSn—Ag—Cu—Bi—In five-element alloy powder, and the composition of thepowder mixture consists of 5-10 mass % of Zn, 0.005-1.5 mass % of Ag,0.002-1.0 mass % of Cu, 0.005-15 mass % of Bi, 0.005-15 mass % of In,and a remainder of Sn.
 9. A solder paste as claimed in claim 1 whereineach powder in the powder mixture is selected from a Sn—Zn binary alloypowder, a Sn—Zn—Sb ternary alloy powder, a Sn—Ag binary alloy powder,and a Sn—Ag—Sb ternary alloy powder, and the composition of the powdermixture consists of 5-10 mass % of Zn, 0.005-1.5 mass % of Ag, 0.005-1.0mass percent of Sb, and a remainder of Sn.
 10. A solder paste as claimedin claim 1 wherein each powder in the powder mixture is selected from aSn—Zn binary alloy powder, a Sn—Zn—Cu ternary alloy powder, a Sn—Zn—Sbternary alloy powder, a Sn—Zn—Cu—Sb quaternary alloy powder, a Sn—Agbinary alloy powder, a Sn—Ag—Cu ternary alloy powder, a Sn—Ag—Sb ternaryalloy powder, and a Sn—Ag—Cu—Sb quaternary alloy powder, and thecomposition of the powder mixture consists of 5-10 mass % of Zn,0.005-1.5 mass % of Ag, 0.002-1.0 mass % of Cu, 0.005-1.0 mass % of Sb,and a remainder of Sn.
 11. A solder paste as claimed in claim 1 whereineach powder in the powder mixture is selected from a Sn—Zn binary alloypowder, a Sn—Zn—Bi ternary alloy powder, a Sn—Zn—Sb ternary alloypowder, a Sn—Zn—Bi—Sb quaternary alloy powder, a Sn—Ag binary alloypowder, a Sn—Ag—Sb ternary alloy powder, and a Sn—Ag—Bi—Sb quaternaryalloy powder, and the composition of the powder mixture consists of 5-10mass % of Zn, 0005-15 mass % of Ag, 0.005-15 mass % of Bi, 0.005-1.0mass % of Sb, and a remainder of Sn.
 12. A solder paste as claimed inclaim 1 wherein each powder in the powder mixture is selected from aSn—Zn binary alloy powder, a Sn—Zn—In ternary alloy powder, a Sn—Zn—Sbternary alloy powder, a Sn—Zn—In—Sb quaternary alloy powder, a Sn—Agbinary alloy powder, a Sn—Ag—In ternary alloy powder, a Sn—Ag—Sb ternaryalloy powder, and a Sn—Ag—In—Sb quaternary alloy powder, and thecomposition of the powder mixture consists of 5-10 mass % of Zn,0.005-1.5 mass % of Ag, 0.005-15 mass % of In, 0005-1.0 mass % of Sb,and a remainder of Sn.
 13. A solder paste as claimed in claim 1 whereineach powder in the powder mixture is selected from a Sn—Zn binary alloypowder, a Sn—Zn—Bi ternary alloy powder, a Sn—Zn—Cu ternary alloypowder, a Sn—Zn—Sb ternary alloy powder, a Sn—Zn—Cu—Bi quaternary alloypowder, a Sn—Zn—Cu—Sb quaternary alloy powder, a Sn—Zn—Bi—Sb quaternaryalloy powder, a Sn—Zn—Cu—Bi—Sb five-element alloy powder, a Sn—Ag binaryalloy powder, a Sn—Ag—Cu ternary alloy powder, a Sn—Ag—Sb ternary alloypowder, a Sn—Ag—Cu—Bi quaternary alloy powder, a Sn—Ag—Cu—Sb quaternaryalloy powder, a Sn—Ag—Bi—Sb quaternary alloy powder, and aSn—Ag—Cu—Bi—Sb five-element alloy powder, and the composition of thepowder mixture consists of 5-10 mass % of Zn, 0.005-1.5 mass % of Ag,0.002-1.0 mass % of Cu, 0.005-15 mass % of Bi, 0.005-1.0 mass % of Sb,and a remainder of Sn.
 14. A solder paste as claimed in claim 1 whereineach powder in the powder mixture is selected from a Sn—Zn binary alloypowder, a Sn—Zn—In ternary alloy powder, a Sn—Zn—Cu ternary alloypowder, a Sn—Zn—Sb ternary alloy powder, a Sn—Zn—Cu—In quaternary alloypowder, a Sn—Zn—Cu—Sb quaternary alloy powder, a Sn—Zn—In—Sb quaternaryalloy powder, a Sn—Zn—Cu—In—Sb five-element alloy powder, a Sn—Ag binaryalloy powder, a Sn—Ag—Cu ternary alloy powder, a Sn—Ag—In ternary alloypowder, a Sn—Ag—Sb ternary alloy powder, a Sn—Ag—Cu—In quaternary alloypowder, a Sn—Ag—Cu—Sb quaternary alloy powder, a Sn—Ag—In—Sb quaternaryalloy powder, and a Sn—Ag—Cu—In—Sb five-element alloy powder, and thecomposition of the powder mixture consists of 5-10 mass % of Zn,0.005-1.5 mass % of Ag, 0.002-1.0 mass % of Cu, 0.005-15 mass % of In,0.005-1.0 mass % of Sb, and a remainder of Sn.
 15. A solder paste asclaimed in claim 1 wherein each powder in the powder mixture is selectedfrom a Sn—Zn binary alloy powder, a Sn—Zn—Bi ternary alloy powder, aSn—Zn—In ternary alloy powder, a Sn—Zn—Sb ternary alloy powder, aSn—Zn—Bi—In quaternary alloy powder, a Sn—Zn—Bi—Sb quaternary alloypowder, a Sn—Zn—In—Sb quaternary alloy powdery a Sn—Zn—Bi—In—Sbfive-element alloy powder, a Sn—Ag binary alloy powder, a Sn—Ag—Internary alloy powder, a Sn—Ag—Sb ternary alloy powder, Sn—Ag—Bi—Inquaternary alloy powder, a Sn—Ag—Bi—Sb quaternary alloy powder, aSn—Ag—In—Sb quaternary alloy powder, and a Sn—Ag—Bi—In—Sb five-elementalloy powder, and the composition of the powder mixture consists of 5-10mass % of Zn, 0.005-1.5 mass % of Ag, 0.005-15 mass % of Bi, 0.005-15mass % of In, 0.005-1.0 mass % of Sb, and a remainder of Sn.
 16. Asolder paste as claimed in claim 1 wherein each powder in the powdermixture is selected from a Sn—Zn binary alloy powder, a Sn—Zn—Bi ternaryalloy powder, a Sn—Zn—In ternary alloy powder, a Sn—Zn—Cu ternary alloypowder, a Sn—Zn—Sb ternary alloy powder, a Sn—Zn—Bi—In quaternary alloypowder, a Sn—Zn—Cu—Bi quaternary alloy powder, a Sn—Zn—Cu—In quaternaryalloy powder, a Sn—Zn—Cu—Sb quaternary alloy powder, a Sn—Zn—Bi—Sbquaternary alloy powder, a Sn—Zn—In—Sb quaternary alloy powder, aSn—Zn—Cu—Bi—In five-element alloy powder, a Sn—Zn—Cu—Bi—Sb five-elementalloy powder, a Sn—Zn—Cu—In—Sb five-element alloy powder, aSn—Zn—Bi—In—Sb five-element alloy powder, a Sn—Zn—Cu—Bi—In—Sbsix-element allay powder, a Sn—Ag binary alloy powder, a Sn—Ag—Cuternary alloy powder, a Sn—Ag—In ternary alloy powder, a Sn—Ag—Sbternary alloy powder, a Sn—Ag—Cu—Bi quaternary alloy powder, aSn—Ag—Cu—In quaternary alloy powder, a Sn—Ag—Bi—In quaternary alloypowder, a Sn—Ag—Cu—Sb quaternary alloy powder, a Sn—Ag—Bi—Sb quaternaryalloy powder, a Sn—Ag—In—Sb quaternary alloy powder, a Sn—Ag—Cu—Bi—Infive-element alloy powder, a Sn—Ag—Cu—Bi—Sb five-element alloy powder, aSn—Ag—Cu—In—Sb five-element alloy powder, a Sn—Ag—Bi—In—Sb five-elementalloy powder, and a Sn—Ag—Cu—Bi—In—Sb six-element alloy powder, and thecomposition of the powder mixture consists of 5-10 mass % of Zn,0.005-15 mass % of Ag, 0.002-1.0 mass % of Cu, 0.005-15 mass % of Si,0.005-15 mass % of In, 0005-1.0 mass % of Sb, and a remainder of Sn. 17.A solder paste as claimed in claim 1, wherein each powder in the powdermixture is selected from a Sn—Zn binary alloy powder, a Sn—Zn—Bi ternaryalloy powder, a Sn—Zn—In ternary alloy powder, a Sn—Zn—Cu ternary alloypowder, a Sn—Zn—Bi—In quaternary alloy powder, a Sn—Zn—Cu—Bi quaternaryalloy powder, a Sn—Zn—Cu—In quaternary alloy powder, a Sn—Zn—Cu—Bi—Infive-element alloy powder, a Sn—Ag binary alloy powder, a Sn—Ag—Cuternary alloy powder, a Sn—Ag—In ternary alloy powder, a Sn—Ag—Cu—Biquaternary alloy powder, a Sn—Ag—Cu—In quaternary alloy powder, aSn—Ag—Bi—In quaternary alloy powder, and a Sn—Ag—Cu—Bi—In five-elementalloy powder, and the composition of the powder mixture consists of 5-10mass % of Zn, 0.005-1.5 mass % of Ag, one or more selected from thegroup consisting of 0.002-1.0 mass % of Cu, 0.005-15 mass % of Bi, and0.005-15 mass % of In, and a remainder of Sn.
 18. A solder paste asclaimed in claim 17 wherein the amount in the powder mixture of a Sn—Agbinary alloy powder and a Sn—Ag based alloy powder comprising Sn, Ag,and at least one additional element is at most 30 mass % of the powdermixture.
 19. A solder paste comprising an alloy powder blended with aflux, wherein the alloy powder is a powder mixture consisting of aplurality of powders each of which is a Sn—Zn based alloy powder notcontaining Ag or a Sn—Ag based alloy powder not containing Zn, each ofthe plurality of powders is selected from a Sn—Zn binary alloy powder, aSn—Zn—Bi ternary alloy powder, a Sn—Zn—In ternary alloy powder, aSn—Zn—Cu ternary alloy powder, a Sn—Zn—Sb ternary alloy powder, aSn—Zn—Bi—In quaternary alloy powder, a Sn—Zn—Cu—Bi quaternary alloypowder, a Sn—Zn—Cu—In quaternary alloy powder, a Sn—Zn—Cu—Sb quaternaryalloy powder, a Sn—Zn—Bi—Sb quaternary alloy powder, a Sn—Zn—In—Sbquaternary alloy powder, a Sn—Zn—Cu—Bi—in five-element alloy powder, aSn—Zn—Cu—Bi—Sb five-element alloy powder, a Sn—Zn—Cu—In—Sb five-elementalloy powder, a Sn—Zn—Bi—In—Sb five-element alloy powder, aSn—Zn—Cu—Bi—In—Sb six-element alloy powder, a Sn—Ag binary alloy powder,a Sn—Ag—Cu ternary alloy powder, a Sn—Ag—Bi ternary alloy powder, aSn—Ag—In ternary alloy powder, a Sn—Ag—Sb ternary alloy powder, aSn—Ag—Cu—Bi quaternary alloy powder, a Sn—Ag—Cu—In quaternary alloypowder, a Sn—Ag—Bi—In quaternary alloy powder, a Sn—Ag—Cu—Sb quaternaryalloy powder, a Sn—Ag—Bi—Sb quaternary alloy powder, a Sn—Ag—In—Sbquaternary alloy powder, a Sn—Ag—Cu—Bi—In five-element alloy powder, aSn—Ag—Cu—Bi—Sb five-element alloy powder, a Sn—Ag—Cu—In—Sb five-elementalloy powder, a Sn—Ag—Bi—In—Sb five-element alloy powder, and aSn—Ag—Cu—Bi—In—Sb six-element alloy powder, and the composition of thepowder mixture consists of 5-10 mass % of Zn, 0.005-1.5 mass % of Ag,0.005-1.0 mass % of Sb, from zero to three elements selected from thegroup consisting of 0.002-1.0 mass % of Cu, 0.005-15 mass % of Bi, and0.005-15 mass % of In, and a remainder of Sn.